Two main types of an energy device are an energy storage device and an energy generating device. Typical examples of the energy storage device are an electrochemical capacitor and a battery, which have already been used in their appropriate markets. Examples of the electrochemical capacitor are: an electric double layer capacitor which uses an activated carbon as a polarizable electrode and utilizes only an electric double layer formed at an interface between a pore surface of the activated carbon and an electrolytic solution; and a redox capacitor which uses a transition metal oxide, such as ruthenium nitrate, whose valence continuously changes, and an electrically-conductive polymer which can be doped. Moreover, two main types of the battery are: a secondary battery which can be charged and discharge by utilizing intercalation and chemical reactions of active materials; and a primary battery which is basically not rechargeable after being discharged once.
The most basic component common to all of these various energy storage devices is an electrode active material which can discharge energy in principle. In addition, to take out the energy stored in the electrode active material, a current collector (electric conductor) is further required, which has electron conductivity and is electrically connected to the electrode active material. Since the current collector needs to transfer the energy of the electrode active material with high efficiency, a metallic material, such as aluminum, copper, or stainless steel, which is very low in resistance is typically used as the current collector. However, in the case of using the electrolytic solution, such as a sulfuric acid aqueous solution, which causes metal to corrode, for example, a rubber-based material to which electrical conductivity is given may be used as the current collector.
As the application of the energy storage device is increasing in recent years, there is a need for the energy storage device which has excellent properties, i.e., which is lower in resistance and can discharge higher current. First, these properties were expected of the electric double layer capacitor which was the lowest in resistance in principle among the energy storage devices, and the electric double layer capacitor having such properties was realized by disposing a carbon-based electrically-conductive layer on a joint surface between the electrode active material and the current collector. Since an electronic resistance in the electrode active material of the electric double layer capacitor is comparatively lower than those of the other secondary batteries, a contact resistance between the electrode active material and the current collector accounts for a nonnegligible percentage with respect to the resistance of a device, so that the carbon-based electrically-conductive layer is disposed on the joint surface. At present, similar technical trend to the above has been pursued for a lithium secondary battery.
To solve the above problems, an energy storage device has been studied which uses as the electrode active material a carbon nanotube whose one end is connected to the current collector (see Patent Document 1 for example). The carbon nanotube is a hollow carbon material having a minimum diameter of 0.4 nm and a maximum length of 4 mm. Unlike conventional pellet electrodes, a carbon nanotube electrode in which one end of the carbon nanotube is connected to a substrate does not require an electric conduction assisting material and a binding material. Therefore, a volume fraction of the active material is 100%. In addition, since the carbon nanotube is connected to the current collector that is the substrate, the carbon nanotube electrode is very low in electrical resistance. Further, the carbon nanotube has an extremely high ideal specific surface area of 2,625 m2/g, and is especially suitable to be applied to the electric double layer capacitor.
However, in the carbon nanotube electrode, the catalyst metal used when synthesizing the carbon nanotube remains on the electric conductor. Therefore, if a voltage is applied to the electrode as the energy storage device, the catalyst metal and a metal constituting the electric conductor are ionized, and flow out to the electrolytic solution. Then, a reaction current flows, and this decreases reliability of the energy storage device. Therefore, it has been extremely difficult to apply the carbon nanotube electrode to the energy storage device having the above structure.
Patent Document 1 describes that in the electric double layer capacitor including the polarizable electrode formed by the carbon nanotube formed in an electrode forming region of the substrate, the carbon nanotube is formed in the electrode forming region except for a predetermined region. This aims to obtain large electric capacity from initial charging and discharging and obtain large electric capacity even at low temperature, by facilitating impregnation of an inside of the carbon nanotube with the electrolytic solution. Patent Document 1 does not describe a structure in which the formed carbon nanotube is toppled to cover the surface of a non-forming region or the decrease in reliability of the energy storage device due to the reaction current generated by the ionization of the metal.
Patent Document 2 describes that in manufacturing not the energy storage device but an electronic device, such as a transistor, a plurality of electrodes are formed on an insulating film disposed on the surface of the substrate, the carbon nanotube is formed on one electrode so as to be vertically aligned, and then the carbon nanotube is toppled toward the other electrode with a base of the carbon nanotube fixed. This aims to surely connect these two electrodes on the substrate by using the carbon nanotube as a wire material extending between the electrodes. Patent Document 2 does not describe the use of the carbon nanotube as the electrode active material. Moreover, Patent Document 2 does not describe the energy storage device. Therefore, Patent Document 2 does not describe the decrease in reliability of the energy storage device due to the reaction current generated in the electrolytic solution by the ionization of the metal.    Patent Document 1: Japanese Laid-Open Patent Application Publication 2005-259760    Patent Document 2: Japanese Laid-Open Patent Application Publication 2006-228818